Switched capacitor charge pumps deliver inductorless voltage boost

One of the first things you learn in electrical engineering school is that you can’t directly step up a DC voltage, but instead you have to first convert to AC. So what do you do when you have a DC supply you need to boost to develop those myriad rails in a design?

You could build an inductor-based oscillator, then rectify and filter the oscillator’s output. However, an inductor is a relatively large and costly discrete component, and the configuration also creates EMI/RFI.

That’s why providers of step-up ICs have devised a clever approach of pumping charge into a capacitor, and then switching its connection to a second capacitor, resulting in an inductorless voltage boost. The topology is smaller than the inductor-based circuit and generates relatively little EMI/RFI.

In brief, this charge-pump technique works by charging a 'flying' capacitor from the DC rail, then switching that capacitor so it is connected to another, already charged 'reservoir' capacitor on the load side. Charge flows from the first capacitor to the second, thus doubling the output voltage according fundamental relationship among charge, voltage and capacitance. The capacitor is then switched back to the supply rail and the cycle continues; the switching rate is from several kHz to a few MHz.

This is a relatively easy and low-cost way to double the supply voltage. Due to the attractiveness of the charge-pump approach, IC vendors such as Linear Technology Corp (now part of Analog Devices) have devoted significant effort to refining and enhancing it, yielding excellent performance in terms of regulation, ripple, efficiency, and other key parameters. Other variations of this topology can even step-down (buck), buck/boost, level-shift, or invert the supply.

Click here to learn more about how these switched-capacitor boost regulators operate, and see the wide variety of ICs available at Linear Technology.